1. Field of the Invention
A toroidal type continuously variable transmission according to the present invention is used, for example, as a speed change unit of a transmission of a motor vehicle or transmissions of various industrial machines, respectively.
2. Related Background Art
It has been investigated that a toroidal type continuously variable transmission schematically shown in FIGS. 24 and 25 is used as a transmission of a motor vehicle. For example, as disclosed in Japanese Utility Model Laid-Open No. 62-71465 (1987), in such a toroidal type continuously variable transmission, an input disc 2 is supported coaxially with an input shaft 1 and an output disc 4 is secured to an end of an output shaft 3 disposed coaxially with the input shaft 1. Within a casing (described later in connection with FIGS. 26 to 28) containing the toroidal type continuously variable transmission, there are provided trunnions 7 rockable around pivot shafts 6 located at positions twisted with respect to the input shaft 1 and the output shaft 3.
That is to say, each trunnion 7 is provided at its both end outer surfaces with the pivot shafts 6 coaxial with each other. Accordingly, the pivot shafts 6 do not intersect with center lines of the discs 2, 4 but extend perpendicular to such center lines. Further, central portions of the trunnions 7 support proximal ends of displacement shafts 8 so that inclination angles of the displacement shafts 8 can be adjusted by rocking or swinging the trunnions 7 around the pivot shafts 6. Power rollers 9 are rotatably supported around the displacement shafts 8 supported by the trunnions 7. The power rollers 9 are interposed between the input disc 2 and the output disc 4. Inner surfaces 2a, 4a of the discs 2, 4 which are opposed to each other have concave surfaces obtained by rotating arcs having centers on the pivot shaft 6 around the input shaft 1 and the output shaft 3. Peripheral surfaces 9a of the power rollers 9 having spherical convex shapes abut against the inner surfaces 2a, 4a. A pressing device 10 of loading cam type is disposed between the input disc 2 and the output disc 4 so that the input disc 2 is can be urged elastically toward the output disc 4 by the pressing device 10. The pressing device 10 comprises a cam plate 11 rotated together with the input shaft 1, and a plurality (for example, four) of rollers 13 held by a holder 12. One side surface (left side surface in FIGS. 24 and 25) of the cam plate 11 is constituted as a cam surface 14 having unevenness or undulation extending along a circumferential direction, and an outer surface (right side surface in FIGS. 24 and 25) of the input disc 2 has a similar cam surface 15. The plurality of rollers 13 are rotatably supported for rotation around axes extending radially with respect to the center line of the input shaft 1.
In use of the toroidal type continuously variable transmission having the above-mentioned construction, when the cam plate 11 is rotated as the input shaft 1 is rotated, the plurality of rollers 13 are urged against the cam surface 15 formed on the outer surface of the input disc 2 by the cam surface 14. As a result, the input disc 2 is urged against the plurality of power rollers 9, and, at the same time, due to the frictional engagement between the pair of cam surfaces 14, 15 and the plurality of rollers 13, the input disc 2 is rotated. Rotation of the input disc 2 is transmitted to the output disc 4 through the plurality of power rollers 9, thereby rotating the output shaft 3 secured to the output disc 4.
Regarding a rotational speed ratio (speed change ratio) between the input shaft 1 and the output shaft 3, when deceleration is effected between the input shaft 1 and the output shaft 3, the trunnions 7 are rocked or swung around the pivot shafts 6 in predetermined directions, thereby inclining the displacement shafts 8 so that the peripheral surfaces 9a of the power rollers 9 abut against a portion of the inner surface 2a of the input disc 2 near the center and a portion of the inner surface 4a of the output disc 4 near its outer periphery, respectively, as shown in FIG. 24. On the other hand, when acceleration is effected, the trunnions 7 are rocked around the pivot shafts 6 in opposite directions, thereby inclining the displacement shafts 8 so that the peripheral surfaces 9a of the power rollers 9 abut against a portion of the inner surface 2a of the input disc 2 near its outer periphery and a portion of the inner surface 4a of the output disc 4 near the center, respectively, as shown in FIG. 25. If the inclination angles of the displacement shafts 8 are selected to an intermediate value between FIG. 24 and FIG. 25, an intermediate speed change ratio can be obtained.
When the actual transmission of the motor vehicle is constituted by the above-mentioned the toroidal type continuously variable transmission, it is well known in the art to provide a so-called toroidal type continuously variable transmission of double cavity type in which two sets of input disc 2, output disc 4 and power rollers 9 are prepared, and such two sets of input disc 2, output disc 4 and power rollers 9 are arranged in parallel to each other along a power transmitting direction. FIGS. 26 to 28 show an example of such a toroidal type continuously variable transmission of double cavity type disclosed in Japanese Patent Publication No. 8-23386 (1996).
An input shaft la is supported within a casing 5 for only rotation. A cylindrical transmission shaft 16 is rotatably supported around the input shaft 1a in coaxial with the latter for rotation relative to the input shaft 1a. First and second input discs 17, 18 corresponding to first and second outer discs of the present invention are supported on both ends of the transmission shaft 16 via ball splines 19 so that inner faces 2a of these discs are opposed to each other. Accordingly, the first and second input discs 17, 18 are rotatably supported within the casing 5 coaxially with each other and rotate synchronously with each other.
Further, first and second output discs 20, 21 corresponding to first and second inner discs of the present invention are supported around an intermediate portion of the transmission shaft 16 via a sleeve 22. An output gear 23 is integrally formed on an outer peripheral surface of an intermediate portion of the sleeve 22, and the sleeve has an inner diameter greater than an outer diameter of the transmission shaft 16. The sleeve is rotatably supported by a support wall 24 provided within the casing 5 via a pair of bearings 25 in such a manner than the sleeve is disposed coaxially with the transmission shaft 16 and can merely be rotated. In this way, the first and second output discs 20, 21 are spline-connected to both ends of the sleeve 22 rotatably mounted around the intermediate portion of the transmission shaft 16 in a condition that inner surfaces 4a of the discs 20, 21 are directed toward opposite directions. Accordingly, the first and second output discs 20, 21 are supported in coaxial with the first and second input discs 17, 18 and are rotated independently from the first and second input discs 17, 18 in a condition that the inner surfaces 4a are opposed to the respective inner surfaces 2a of the first and second input discs 17, 18.
Further, two pairs of yokes 26a, 26b are supported by an inner wall of the casing 5 at both sides of the first and second output discs 20, 21 with the interposition of these output discs 20, 21. The yokes 26a, 26b correspond to yokes constituting first and second supporting means of the present invention and are formed as rectangular frames, respectively, by press-working a metal plate such as steel or forging metal material such as steel. The yokes 26a, 26b are provided at their four corners with circular support holes 31 for rockably supporting first and second pivot shafts 29, 30 provided on both ends of first and second trunnions 27, 28 (described later) and are also provided with circular locking holes 32 formed in central portions of the yokes in a width-wise direction (left-and-right direction in FIGS. 27 and 28) thereof at both ends of the transmission shaft 16 in an axial direction (left-and-right direction in FIG. 26) thereof. The pairs of yokes 26a, 26b each having the above-mentioned configuration are supported by support posts 33a, 33b formed on opposed portions of the inner wall of the casing 5 for slight displacement. The support posts 33a, 33b are opposed to each other and are disposed within a first cavity 34 between the inner surface 2a of the first input disc 17 and the inner surface 4a of the first output disc 20 and a second cavity 35 between the inner surface 2a of the second input disc 18 and the inner surface 4a of the second output disc 21. Accordingly, in a condition that the yokes 26a, 26b are supported by the support posts 33a, 33b, first ends of the yokes 26a, 26b are opposed to an outer peripheral portion of the first cavity 34 and the other ends are opposed to an outer peripheral portion of the second cavity 35.
Further, a pair of first trunnions 27 are disposed within the first cavity 34 at diametrically opposed positions of the first input disc 17 and the first output disc 20, and a pair of second trunnions 28 are disposed within the second cavity 35 at diametrically opposed positions of the second input disc 18 and the second output disc 21. As shown in FIG. 27, the four (in total) first pivot shafts 29 which are coaxially provided on both ends of the trunnions 27 (two in each trunnion) are supported by the first ends of the pair of yokes 26a, 26b for rocking movement and axial displacement. That is to say, the first pivot shafts 29 are supported within the support holes 31 formed in the first ends of the yokes 26a, 26b via radial needle bearings 36. Each of the radial needle bearings 36 has an outer race 37 having a spherical convex outer peripheral surface and a cylindrical inner peripheral surface, and a plurality of needles 38. Accordingly, the first pivot shafts 29 are supported at both axial sides on the first ends of the yokes 26a, 26b for reversible rocking movement and axial displacement. Further, as shown in FIG. 28, the four (in total) second pivot shafts 30 which are coaxially provided on both ends of the second trunnions 28 (two in each trunnion) are supported within the second cavity 35 in the same manner as the first pivot shafts 29 provided on the first trunnions 27.
The first and second trunnions 27, 28 supported within the casing 5 for rocking movements and displacements in axial directions of first and second pivot shafts 29, 30 in this way are provided at their intermediate portions with circular holes 39, as shown in FIGS. 27 and 28. The first and second displacement shafts 40, 41 are supported in these circular holes 39. The first and second displacement shafts 40, 41 have support shaft portions 42 parallel with and eccentric with each other, and pivot shaft portions 43. The support shaft portions 42 are rotatably supported within the circular holes 39 via radial needle bearings 44. Further, first and second power rollers 45, 46 are rotatably supported around the pivot shaft portions 43 via other radial needle bearings 47.
Incidentally, the pair of first and second displacement shafts 40, 41 provided for each of the first and second cavities 34, 35 are disposed at opposite directions (diametrically opposed at 180 degrees) with respect to the input shaft 1a and the transmission shaft 16 for each of the first and second cavities 34, 35. Further, directions along which the pivot shaft portions 43 of the first and second displacement shafts 40, 41 are offset (eccentric) from the support shaft portions 42 are the same (up-and-down opposite directions in FIGS. 27 and 28) with respect to the rotational direction of the first and second input and output discs 17, 18, 20, 21. Further, the eccentric directions are substantially perpendicular to an installation direction of the input shaft 1a. Accordingly, the first and second power rollers 45, 46 are supported for slight displacement in the installation direction of the input shaft 1a and the transmission shaft 16 (slight axial displacement). As a result, if the first and second power rollers 45, 46 tend to be displaced in the axial direction of the input shaft 1a and the transmission shaft 16 (left-and-right direction in FIG. 26, and, direction perpendicular to the planes of FIGS. 27 and 28) by change in elastic deformation amount of structural parts due to fluctuation in torque to be transmitted by the toroidal type continuously variable transmission, such displacement can be absorbed without acting any excessive stress on the structural parts.
Further, between outer surfaces of the first and second power rollers 45, 46 and inner surfaces of intermediate portions of the first and second trunnions 27, 28, there are provided, in order from the outer surfaces of the first and second power rollers 45, 46, thrust ball bearings 48, and thrust bearings 49 such as sliding bearings or needle bearings. The thrust ball bearings 48 serve to support thrust load acting on the first and second power rollers 45, 46 and to allow rotations of the first and second power rollers 45, 46. Further, the thrust bearings 49 serve to support thrust loads acting on outer races 50 of the thrust ball bearings 48 and to allow the pivot shaft portions 43 and the outer races 50 to rock around the support shaft portions 42.
Further, drive rods 51 are connected to one end (lower ends in FIGS. 27 and 28) of each of the first and second trunnions 27, 28, and drive pistons 52 are secured to outer surfaces of intermediate portions of the drive rods 51. The drive pistons 52 are slidably mounted within drive cylinders 53 in an oil-tight fashion. The drive pistons 52 and the drive cylinders 53 constitute actuators for displacing the first and second trunnions 27, 28 along the axial directions of the first and second pivot shafts 29, 30. Further, pressurized oil can be supplied within the drive cylinders 53 in response to switching of a control valve (not shown).
Further, an pressing device 10 of loading cam type is disposed between the input shaft 1a and the first input disc 17. The pressing device 10 includes a cam plate 11 spline-connected to the intermediate portion of the input shaft 1a so that it can be rotated together with the input shaft 1a but cannot be displaced in the axial direction, and a plurality of rollers 13 rotatably held by a holder 12. When the input shaft 1a is rotated, the pressing device serves to rotate the first input disc 17 while urging it toward the second input disc 18.
When the toroidal type continuously variable transmission having the above-mentioned construction is driven, the rotation of the input shaft 1a is transmitted to the first input disc 17 through the pressing device 10, so that the first and second input discs 17, 18 are rotated in synchronous with each other. The rotation of the first and second input discs 17, 18 is transmitted to the first and second output discs 20, 21 through the pairs of first and second power rollers 45, 46 disposed within the first and second cavities 34, 35. The rotation of the first and second output discs 20, 21 is picked-up by the output gear 23. When the rotational speed ratio between the input shaft 1a and the output gear 23 is changed, by switching the control valve, the pairs of drive pistons 52 corresponding to the first and second cavities 34, 35 are displaced in opposite directions by the same distance for the cavities 34, 35, respectively.
When the drive pistons 52 are displaced, two pairs (four in total) of trunnions 27, 28 are displaced in opposite directions, so that, for example, the first and second power rollers 45, 46 at the right in FIGS. 27 and 28 are shifted downwardly (FIGS. 27 and 28) and the first and second power rollers 45, 46 at the left in FIGS. 27 and 28 are shifted upwardly (FIGS. 27 and 28). As a result, directions of tangential forces acting on the contact areas between the peripheral surfaces 9a of the first and second power rollers 45, 46 and the inner surfaces 2a, 4a of the first and second input discs 17, 18 and the first and second output discs 20, 21 are changed. As the directions of forces are changed, the first and second trunnions 27, 28 are rocked in opposite directions around the first and second pivot shafts 29, 30 supported by the yokes 26a, 26b. As a result, as shown in FIGS. 24 and 25, the contact areas between the peripheral surfaces 9a of the first and second power rollers 45, 46 and the inner surfaces 2a, 4a of the discs 17, 18, 20, 21 are changed, thereby changing the rotational speed ratio between the input shaft 1a and the output gear 23.
In the conventional arrangement shown in FIGS. 26 to 28, the first and second trunnions 27, 28 are supported within the casing through the support posts 33a, 33b and the yokes 26a, 26b. Thus, since the number of parts is increased, not only manufacture, control and assembling of the parts become troublesome, but also height of the toroidal type continuously variable transmission in the up-and-down direction in FIGS. 26 to 28 is increased, so that it is hard to make the transmission compact and light-weight. Further, if the transmission is forcibly made compact and light-weight to permit installation of the transmission within a limited space, strength of parts is decreased, thereby worsening endurance.
Japanese Patent Laid-Open No. 10-274300 (1998) discloses an arrangement in which pivot shafts provided on both ends of trunnions in a toroidal type continuously variable transmission are supported by support members directly secured to an inner surface of a casing. With this arrangement, since the number of parts is decreased, the transmission can be made compact and light-weight. However, in case of the toroidal type continuously variable transmission disclosed in this document, the support members for supporting the pivot shafts provided on both ends of the trunnions are independently provided for each trunnion.
Thus, in the arrangement disclosed in the above Japanese Patent Laid-Open No. 10-274300, loads acting on the trunnions when the toroidal type continuously variable transmission is driven directly act on the casing. More specifically, when the toroidal type continuously variable transmission is driven, since pressure acting on contact areas between inner surfaces of input and output discs and peripheral surfaces of power rollers is great, the power rollers are subjected to great thrust loads. Such thrust loads act on the support portions for the pivot shafts provided on both ends of the trunnions through the trunnions. In the arrangement disclosed in above-mentioned document, the great loads acting on the pivot shafts in this way act on the casing as they are. In many cases, since the casing of the transmission is made of light alloy such as aluminium alloy to reduce the weight, in order to prevent displacement of the pivot shafts and to ensure the endurance of the casing regardless of great loads, it is necessary to increase a wall thickness of the casing, with the result that it is hard to make the transmission compact and light-weight.
Further, when the toroidal type continuously variable transmission is driven, due to the great loads acting on the trunnions from the power rollers, the trunnions are elastically deformed so that the inner surfaces thereof becomes concave. As a result, parallelism between central axes of the pivot shafts provided on the ends of the trunnions and central axes of circular holes formed in the support members secured to the inner surface of the casing is lost more or less. In the arrangement disclosed in above-mentioned document, it is not considered that the trunnions can be displaced smoothly without damaging any parts even if such a condition occurs.